In Ethernet (registered trademark) devices, pulse transformers are used for the purpose of maintaining impedance matching and electrical isolation at the input/output terminals. Inside this kind of transformer, a soft magnetic material is typically used as the magnetic core. Moreover, it is desirable that in a temperature range of −40 to 85° C. and under application of a direct current magnetic field, the pulse transformer have high incremental permeability μΔ, such as regulated by US standards ANSI X3.263-1995[R2000].
Through the recent advancement of communication technology, not only are transmission speeds becoming faster in Ethernet (registered trademark) devices, but there is also a trend of directly supplying drive power for the device together with the transmission signal. In this case, larger current is applied to and used in the pulse transformer than in the past. Moreover, because surrounding parts inside the transform become hot due to large current, the magnetic core of the pulse transformer is used in a higher temperature condition. Therefore, for MnZn ferrites that are used in the magnetic core of a pulse transformer, it is desired that high impedance, or in other words, high incremental permeability μΔ be maintained when a direct current magnetic field that is higher than in the aforementioned standards is applied. The incremental permeability μΔ is a value that indicates the ease of magnetizing the magnetic core in a state where a magnetic field is applied.
In JP 2004-196632, a technique is disclosed whereby magnetic properties are improved under conditions of high temperature by including a cobalt oxide in the MnZn ferrite. However, in the past, the composition of the MnZn ferrite for a pulse transformer has been designed to obtain a high initial permeability μi so the saturation magnetic flux density is low and, therefore, it was not possible to obtain sufficient incremental permeability μΔ under conditions of high temperature and a high magnetic field.
In JP 7-297020 it is disclosed that a reduction of phosphorous and boron is effected in improving the incremental permeability μΔ. However, the MnZn ferrite disclosed in JP 7-297020 has a composition that was selected for the purpose of reducing iron loss and increasing the effective permeability at 100° C., and because the initial permeability μi at room temperature or lower, which is not mentioned in the embodiments, is too low, it is difficult to hope for an incremental permeability μΔ that is sufficiently satisfactory in low-temperature conditions.
In JP 2006-213532, JP 2001-64076 and JP 2005-179092, techniques are disclosed that regulate the aforementioned impurities. In JP 2006-213532, a technique is disclosed that improves the iron loss and amplitude ratio permeability at 100° C. or lower by regulating the contained amount of chlorine. However, it was impossible to obtain an incremental permeability μΔ of 200 or greater at 23° C. by regulating just the contained amount of chlorine.
In JP 2001-64076, a technique is disclosed that improves electrical power loss by regulating the contained amount of sulfur. However, it was impossible to obtain an incremental permeability μΔ of 200 or greater at 23° C. by regulating just the contained amount of chlorine.
In JP 2005-179092, a technique is disclosed that suppresses abnormal grain growth of ferrite and prevents adverse effects on the properties of the ferrite by regulating the contained amount of phosphorous, boron, sulfur and chlorine. With this technique, MnZn ferrite having a high specific resistance and low squareness ratio is obtained. However, the incremental permeability μΔ under the condition of a high magnetic field could not be called sufficient.
It could therefore be helpful to provide MnZn ferrite having a high incremental permeability μΔ in a wide range of temperatures, and particularly in conditions of high temperature and high magnetic field, as well as a transformer magnetic core that is manufactured using that MnZn ferrite.